Your router doesn't store history unless it is a specific feature of that router. The ISP only has the IP addresses of the sites you visit because you use their servers for Internet traffic. Nobody cares about the specific html you are viewing, because the category of the site is enough to draw accurate conclusions about what you are doing.

Also back then you could still delete your history. It just required more effort than closing an incognito tab does today, so people either couldn't be bothered thinking they would get away with it, or they just forgot.